I connected the belcat into a TDA7052 amp with a MPF102 buffer with input impedance of 3.3M. The amp is working well with electric guitar or acoustic guitar with preamplifier, but from the belcat i get no sound. And i got at the output of the amp a weird noise, like a buzz comes up from a deep well.What is going on?

OK, after all i got output signal, but it just doesn't sounds. It can't be heard. Many people commented for this belcat product that it needs a preamp. So i think i figure out the problem.
But all the piezzos need a preamp? Another friend made an lm386 amp an used as an input a piezo from a vacum and it was sounding loud and clear enough.